Currently, in the field of security systems, motion detection components are generally provided to detect intruders. Intruders may attempt to sabotage or tamper with the motion detection components through various techniques. For example, intruders may attempt to mask detectors by coating them with an opaque substance that acts as a barrier between a motion detection sensor and the corresponding monitored space. Alternatively, intruders may attempt to cover the entire motion detector with an object or otherwise tamper with the motion detection components. Accordingly, security systems having motion detection components are often equipped with an anti-masking system that detects tampering with the motion detection components.
One example of such a system is disclosed in U.S. Patent Application Publication US 2005/0141345, which discloses a method and system for monitoring objects having chips attached. The chips transmit ultrasound signals and include a sabotage sensor 110, a motion detector 130, and a timer 120. The sabotage sensor 110 is activated if removed from the object to which it was attached.
U.S. Pat. No. 4,636,774 discloses a light control system including a variable sensitivity motion detector. When initial entry motion is detected, the system turns on the lights and increases sensitivity to detect continued presence within a room. The increased sensitivity is maintained for a specified period of time while the lights are on. After a period without motion detection, the system extinguishes the lights and sensitivity is reset to a lower value.
U.S. Pat. No. 6,351,234 shows a combination passive microwave and infrared motion detector with anti-masking evaluation. The detector samples sensor signals and compares the signals to a series of possible outcomes. Some of the possible outcomes represent masking conditions. When a person approaches the sensor and is within a predetermined distance of the sensor, the system will automatically initiate an anti-masking algorithm.
Currently available anti-masking systems may generate false alarms if they are too sensitive, as they may be triggered by background noise sources such as insects or birds even when no intruder is present. In some cases, anti-masking systems may generate a false alarm due to external infrared (IR) light sources, IR remote controls, fluorescent lights, or PDAs. Some attempts have been made to reduce the false alarm rate of anti-masking systems.
For example, U.S. Pat. No. 6,380,882 discloses a motion detector that includes a sabotage monitoring device. The motion detector also includes distance independent suppression of signals triggered by small animals and insects to reduce the false alarm rate of the motion detection system.
However, a further need exists to reduce false alarms while simultaneously increasing sensitivity of the anti-masking device when an intruder is present in the monitoring area.